The present invention relates to a magnetic resonance imaging apparatus equipped with a magnet apparatus, preferably used to form a homogeneity magnetic field in a space, wherein magnetic field generation sources are arranged face to face with each other; and a magnet apparatus used as a magnetic field generation source therefor.
A magnetic resonance imaging apparatus provides an image representing the physical and chemical properties of a test object (sample), using the nuclear magnetic resonance that occurs when a high frequency pulse is applied to the test object placed in a homogeneity static magnetic field space. It is mainly used for medical services. The magnetic resonance imaging apparatus (also called the MRI) commonly comprises a magnetic field generation source for applying a homogeneity static magnetic field in the imaging area where the test object is brought, an RF coil for applying a high frequency pulse to the imaging area, a receiving coil for receiving a response from the imaging area, and a gradient coil for applying a gradient magnetic field in the imaging area.
One of the requirements to improve the image quality of a magnetic resonance imaging apparatus is to improve the homogeneity of static magnetic field. To make homogeneity the static magnetic field applied to the imaging area from the magnetic field generation source, the magnetic apparatus used in the magnetic resonance imaging apparatus has the magnetic field adjusted in each phase of designing, assembling and installation.
In the adjustment of the magnetic field in the phase of designing, the magnetic field is made homogeneity, for example, by changing the magnetic properties of the magnetic pole material, depending on the place where the magnetic pole of the magnetic field generation source is machined. The Japanese Application Patent Laid-open Publication No. 2002-253530 discloses the method of generating a homogeneity magnetic field by a magnetic pole provided with a concentric annular cavity. The Japanese Application Patent Laid-open Publication No. Hei 05-243037 discloses the method of changing the thickness of the magnetic pole that changes the saturated magnetization characteristic of a magnetic substance, according to the center and periphery of the magnetic pole surface, whereby a homogeneity magnetic field is created. Japanese Application Patent Laid-open Publication No. 2003-24299 discloses the method of forming a groove and projection on the surface of the magnetic pole, thereby generating a homogeneity magnetic field.
The methods described in Japanese Application Patent Laid-open Publication No. 2002-253530, Japanese Application Patent Laid-open Publication No. Hei 05-243037 and Japanese Application Patent Laid-open Publication No. 2003-24299, however, are the methods for magnetic field adjustment in the so-called phase of designing. These methods fail to create the homogeneity magnetic field by correcting the components of poor homogeneity resulting the production error and ambient environment. This requires final adjustment of the magnetic field to be made in the phase of assembling and installation.
The magnetic field in the phase of assembling is adjusted by correcting the components of poor homogeneity resulting the production error and ambient environment for example, by additionally distributing the magnetic field homogeneity adjusting members composed of a magnetic substance and a permanent magnet, to the magnet apparatus or by removing the magnetic field homogeneity adjusting member. The magnetic field homogeneity adjusting member is commonly arranged in the space sandwiched between the opposing magnetic field generation sources and the gradient coil arranged inside (in the imaging area). Based on this arrangement, Japanese Application Patent Laid-open Publication No. 2001-78984 discloses the method of arranging the magnetic field adjusting members at an interval smaller than the magnetic field adjusting member, thereby ensuring high-precision adjustment of the magnetic field.
Magnetic field adjustment in the phase of installation is the same as that in the phase of assembling. A design is preferably worked out to ensure that each adjustment of magnetic field can be made by one and the same magnetic field adjusting mechanism. Another adjusting mechanism is provided in some cases for improvement of workability. Japanese Application Patent Laid-open Publication No. 2002-238874 discloses a method of using a superconducting coil as a magnetic field generation source. A magnetic field homogeneity adjusting mechanism is provided in the space adjacent to the coil container for storing this superconducting coil, thereby enhancing the efficiency of the magnetic field adjustment. Japanese Application Patent Laid-open Publication No. 2001-224570 introduces a method of forming a radiant groove on a piece of magnetic pole and inserting into this groove a non-magnetic rod with a magnetic field homogeneity adjusting member made of a magnetic substance arranged thereon, whereby the efficiency of magnetic field adjacent is improved. Japanese Application Patent Laid-open Publication No. 2003-153879 describes the method of forming a hole on the gradient coil, which is used as a magnetic field homogeneity adjusting mechanism.
Japanese Application Patent Laid-open Publication No. 2002-165773 describes an MRI apparatus wherein multiple shim holes of different sizes are formed on a shim plate, and a plurality of different shim materials are combined and inserted into the shim holes.